1. Field of the Invention
The present invention relates to a nitride semiconductor substrate and, more specifically, to a gallium nitride (GaN) substrate.
2. Description of the Background Art
Nitride semiconductor substrates such as GaN substrates having energy band-gap of 3.4 eV and high thermal conductivity are attracting attention as material for semiconductor devices including short-wavelength optical devices and power electronic devices. A GaN substrate as an example of such nitride semiconductor substrates has an orientation flat (hereinafter also denoted as OF) or a notch formed at a part of outer circumference, to facilitate positioning or determination of crystal orientation (see, for example, Japanese Patent Laying-Open Nos. 2008-42157 and 2002-356398).
According to the documents mentioned above, an OF or a notch is formed in a GaN substrate having a (0001) plane as a main surface. A method of forming an OF or a notch in a nitride semiconductor substrate having a main surface inclined from the (0001) plane, however, has not been known. Without forming an OF or a notch, it is difficult to know the plane direction or polar face and, therefore, it may be possible that semiconductor devices are formed in an unintended direction. Therefore, it has been difficult to form semiconductor devices using a nitride semiconductor substrate having a main surface inclined from the (0001) plane.